In recent years, significant efforts have been devoted to fabricate flexible and planar electrodes for supercapacitors to meet the requirements of burgeoning modern-electronic industries and their demands.
Carbon based materials and their derivatives have been widely studied for their feasibility as flexible and free-standing electrode materials. For example, carbon nanotubes (CNT) have been introduced to form free-standing and flexible electrode materials. However, these carbon nanotubes tend to suffer from poor dispersion when used to produce the electrodes.
Graphene, another derivative of carbon, effectively circumvents the abovementioned difficulty to achieve highly stable dispersion of CNT but the mass of their electrodes (even if on a unit area basis) tends to be relatively low (0.5 mg cm−2) and this leads to low capacitance per unit area, otherwise known as low areal capacitance.
Several strategies have been proposed to address the aforementioned limitations. One of the proposals includes the use of physically blended graphene-cellulose paper which showed areal capacitance as high as 81 mF cm−2. Further, the areal capacitance may be improved to 94.5 mF cm−2 by producing self-stacked solvated graphene paper.
More recently, it has been shown that the incorporation of pseudocapacitive materials, such as manganese oxide (MnO2), vanadium oxide (V2O5), vanadium nitride (VN), zinc oxide (ZnO) core-shell configurations, tungsten oxide/molybdenum oxide (WO3−x/MoO3−x) etc., with carbon based materials (e.g. reduced-graphene oxide (rGO) or carbon fabric), improves the electrochemical capacitance due to the electrochemical coupling between the electrochemical double layer capacitance (EDLC) and the pseudocapacitive materials.
In another recent development, it has been demonstrated that areal capacitance as high as 897 mF cm−2 may be achieved using rGO—MnO, paper electrode (half-cell configuration) with a relatively high loading mass of about 3.7 mg cm−2. These electrodes with relatively high loading mass are capable of meeting the requirements for practical applications.
Despite the advantages associated with carbon based materials and their transition metal oxide/nitride composites, their performance may be deleteriously affected by various factors. For example, irreversible adsorption of solvated ions near the inner Helmholtz, layer may occur and this restricts the accessible active surface area. In another example, the stacking of graphene oxide nanosheets caused by strong interlayer interaction tends to limit the complete conversion to rGO and this may lead to the deposition of dense or thick metal oxide layers, thereby preventing diffusion of electrolyte ions and resulting in poor conductivity.
While transition metal oxides may be known for their superior electrochemical properties and versatile preparation routes, typical metal oxide nanostructures tend to lack mechanical flexibility due to their high coefficient of stiffness (Y=49 GPa, δ=200 MPa). To address this, free-standing V2O5 nanofibers have been explored while taking inter-fiber hydrogen bonding (H—H) into consideration. However, the desired mechanical flexibility (Y=24 GPa) remains unachievable.
Among the various known transition metal oxides, molybdenum trioxide (MoO3) happens to be one of the more prominent material due to its high theoretical specific capacity (1117 mA h g−1) and high specific energy density (750 Wh kg−1). It has been explored for use as a prospective cathode as well as an anode material for Lithium ion (Li-ion) batteries. MoO3 has also gained significant interest as a pseudocapacitive electrode material due to its high theoretical specific capacitance value (2700 F g−1), which is associated with the fast faradic redox reactions facilitated by the solid-state diffusion of electrolyte ions (intercalation pseudocapacitance). In one example, free-standing flexible anode materials using WO3−x/MoO3−x core-shell nanowires on carbon fabric have been designed for asymmetric supercapacitors and an areal capacitance of 216 mF cm−2 was attained.
However, the metal oxide based free-standing electrodes as reported above tend to either suffer from low loading mass or rely on electrochemical/electronic active current collectors (e.g. carbon fabric, stainless steel, gold coated alumina membrane, etc.). Hence, there is a need to circumvent or ameliorate these limitations.
Accordingly, the formation of free-standing transition metal oxide electrodes without the need for a current collector (i.e, substrate free) is attractive for emerging deformable and conformable supercapacitors or batteries applications. This has been attempted by using a free-standing carbon black (20 wt %)-MoO3 (80 wt %) nanofiber composite as the anode for Li-ion batteries. A high specific discharge capacity of 800 mA h g−1 was attained. However, the prepared composite electrode failed to demonstrate flexibility and suffered from cracking of the nanofibers. The nanofibers were also restricted to a confined degree of orientation. In another study, graphene nanoflakes (30 wt %) with MoO3 nanobelts (70 wt %) were used for forming a flexible cathode material in rechargeable bendable Li-ion batteries.
Based on the existing studies, conventional strategies to form free-standing flexible MoO3 paper electrodes require conductive fillers, such as the carbon or graphene based materials, at a minimum amount of 20 wt % in order to achieve a reasonable level of conductivity.
Thus, there is a further need to provide for a free-standing flexible electrode with improved conductivity, areal capacitance, mechanical strength and loading mass that does not require support from carbon based current collectors or other substrates. There is also a need to provide for a method for producing such electrodes using lower amounts of conductive fillers. This method needs to be capable of enabling the electrode to harness its pseudocapacitive property.